The demand for wireless communications services is on the rise. Cellular telephones are becoming as common as office and residential telephones. Personal Communications Service (PCS) offers the potential to improve existing cellular communications dramatically. Wireless high-speed broadband packet services hold promise for a whole host of new services for work-at-home, telecommuting, local and Internet access.
As a result of the increased popularity of wireless communications services, bandwidth demands for wireless systems providing these services have increased accordingly. Bandwidth refers to the difference between the two limiting frequencies of a band expressed in Hertz (Hz). Bandwidth is a key limiting factor in determining the capacity of the system. The term "capacity" loosely refers to the number of users a system can service, or the amount of information a system can transmit to a user at any one time.
A traditional solution for increasing the bandwidth efficiency in wireless systems is through frequency reuse. Frequency reuse refers to reusing a common frequency band in different cells within the system. The concept of frequency reuse will be discussed in more detail with reference to FIGS. 1 and 2.
FIG. 1 is a diagram of a typical wireless communication system suitable for practicing the present invention. FIG. 1 shows a base station 20 in wireless communication with terminal stations 22. Base station is connected to a Mobile Switching Center (MSC) 26. MSC 26 can be connected to a fixed network, such as the public switched telephone network (PSTN) 24 shown in FIG. 1 or the Internet (not shown). MSC 26 may also be connected to other base stations (not shown). Terminal stations 22 can be either fixed or mobile.
Base station 20 communicates information to/from terminal stations 22 using radio signals transmitted over a range of carrier frequencies. Frequencies represent a finite natural resource, and are in high demand. Moreover, frequencies are heavily regulated by both Federal and State governments. Consequently, each cellular system has access to a very limited number of frequencies. Accordingly, wireless systems attempt to reuse frequencies in as many cells within the system as possible.
To accomplish this, a cellular system uses a frequency reuse pattern. A frequency reuse pattern is determined by taking the total frequency spectrum allotted to the system and dividing it into K sets of frequencies, with each cell having access to one set of frequencies. For example, if the system were allocated 70 MHZ of frequency spectrum, and there were 7 sets of frequencies (K=7), each set would include 10 MHZ worth of the 70 MHZ available to the system (assuming a uniform distribution). Thus, each cell would have access to 10 MHZ Worth of the total frequency spectrum allotted to the system.
FIGS. 2(A) through 2(D) illustrate examples of frequency reuse patterns corresponding to K=4, 7, 12 and 19, respectively. A cellular communication system has a number of communication sites located throughout the geographic area served by the system. As shown in FIGS. 2(A) through 2(D), a geographic area can be organized into cells and/or sectors, with each cell typically containing a plurality of communication sites such as a base station and terminal stations. A cell is represented in FIGS. 2(A) through 2(D) as a hexagon. FIG. 2(A) shows a frequency reuse pattern where K=4. Cells are placed into groups of four (referred to as a "cluster"), with each cluster employing one of the frequency sets 1 through 4 (the number within each cell in FIG. 2(A) represents a set of frequencies). This cluster of four cells is then repeated until the entire service area is covered. This same pattern is shown in FIGS. 2(B), 2(C) and 2(D) for clusters of 7, 12 and 19 cells, respectively.
In view of the above, it can be appreciated that the larger the frequency reuse factor (i.e., K), the smaller amount of frequency is available to each cell. This limits the overall capacity of the system, for example, by limiting the number of terminal stations that can communicate within a cell, or the data rates at which each terminal station can send and receive information. Consequently, to the extent that the frequency reuse factor (i.e., K) can be lowered, the more capacity the system has to offer wireless communications services.
A major factor in designing a frequency reuse pattern is the attempt to maximize system capacity while maintaining an acceptable signal-to-interference ratio (SIR). SIR refers to the ratio of the level of the received desired signal to the level of the received undesired signal. Most of the undesired signal is due to co-channel interference. Co-channel interference is interference due to the common use of the same frequency band by two different cells. Thus, to a large extent, co-channel interference determines how often a set of frequencies may be reused throughout the entire system. Accordingly, to the extent co-channel interference can be minimized, the lower the frequency reuse factor (i.e., K).
In an attempt to minimize co-channel interference, conventional systems have separated each cell into multiple sectors, with each sector having a directional antenna co-located with a base station at the center of the cell. The beamwidth of each base station antenna is normally wide enough to cover the whole sector. Typically, a cell is divided into three sectors with each sector having a 120 degree antenna. These cell configurations, however, are unsatisfactory for a number of reasons, which are described with reference to FIGS. 3, 4, and 5.
FIG. 3 is a diagram of a conventional three-sector cell. The cell is represented as a hexagon, with solid lines representing hypothetical cell contours. The term "cell contours" refers to the outline of a cell as well as the lines within a cell which define a sector. The cell has a base station located at the center of the cell, with a directional antenna covering each sector of the cell. The beamwidth of each base station antenna is 120 degrees. The analytical cell contours (i.e., coverage area for each antenna transmission beam) is represented by non-solid lines. As shown in FIG. 3, the analytical cell contours for the front lobe of the antenna transmission beam for sector 1 ("originating sector") forms the shape of an oval. The hypothetical cell contours for sector 1, however, forms the shape of a diamond. As a result, it can be appreciated that the analytical cell contours do not match the hypothetical cell contours. Consequently, the analytical cell contours overlap into the sectors 2 and 3 which are adjacent to sector 1, thereby interfering with the antenna transmission beams for sectors 2 and 3. This type of cell configuration is referred to hereinafter as a wide-beam trisector cell (WBTC).
FIG. 4 illustrates a frequency reuse pattern with K=4 using conventional three-sector cells. Cells are placed into a cluster of four, with each cluster employing one of the frequency sets 1 through 4. This cluster of four cells is then repeated until the entire service area is covered. It can be appreciated that if each cell is a WBTC, the analytical cell contours illustrated in FIG. 3 will overlap not only with sectors adjacent to the originating sector within the same cell, but also to sectors adjacent to the originating sector in other cells (hereinafter collectively referred to as "adjacent sectors"). The increased interference caused by overlapping transmissions increases the frequency reuse factor, thereby decreasing overall cell and system capacity. FIG. 4 also illustrates that the distance (D) between each base station using conventional three-sector cells is 3.5 times the radius (R) of each cell.
FIG. 5 shows the cellular system using the frequency reuse pattern described with reference to FIG. 4 with an additional tier of cells creating co-channel interference for a sector labeled sector 4 located in the middle of the diagram ("FIG. 5 target sector"). FIG. 5 shows the FIG. 5 target sector receiving co-channel interference from four cells, labeled A, B, C and D. This interference, in part, explains why the frequency reuse factor for a conventional system using three sectors is typically K=7.
In light of the foregoing, it can be appreciated that a substantial need exists for a cellular architecture that minimizes the co-channel interference for adjacent sectors and cells, thereby decreasing the frequency reuse factor for a system, thereby increasing the overall capacity of the system.